NMR imaging apparatus

ABSTRACT

An NMR imaging apparatus of the present invention which is provided with an improved data acquiring means 16 and which is useful for enhancement of the speed of reconstruction of an image in NMR imaging by a multislice multiecho method is characterized in that a raw data memory 18 which is provided for storing raw data for the maximum number of slices that can be acquired in one scanning separately from the memory 10 of a computer system 6, and an address converter 19 for converting the addressses of the measured data supplied subsequently in accordance with the sequence of the multislice multiecho method and for storing the measured data in the raw data memory 18 in the arrangement different from the order of acquisition are disposed in a data acquiring device, so that a data block for each slice is formed in the raw data memory 18.

TECHNICAL FIELD

The present invention relates to an NMR imaging apparatus which isprovided with an improved data acquiring device and which is useful forenhancement of the speed of reconstruction of an image.

BACKGROUND OF ART

A conventional NMR imaging apparatus is, as shown in FIG. 4, composed ofa static magnetic field coil 2 which is urged by a power source anddriver 1 for generating a uniform and stable static magnetic field; aprobe head (RF coil) 4 which is urged by the power source and driver 1for generating an RF pulse, and which detects an NMR signal of an objectto be examined and supplies it to a preamplifier and detector 3; agradient magnetic field coil 5 which is urged by the power source anddriver 1 for generating linear gradient magnetic fields in the threedirections of x, y and z which overlap the static magnetic field; an A/Dconverter 14 for converting an output signal of the preamplifier anddetector 3 into digital data; and a computer system 6 for controllingthe power source and driver 1 and the preamplifier and detector 3 andfor processing the digital data supplied from the A/D converter 14. Thecomputer system 6 is composed of a central processing unit (CPU) 7, asequence controller 8, an image display (CRT) 9, a memory (DISK) 10, anarray processor (AP) 11 provided with a high-speed memory, aninput/output device (I/O) 12, a system bus 13 for connecting thesemembers 7 through 12 to each other, and an A/D converter 14 connected tothe I/O 12.

FIG. 5 schematically shows the relationship between a slice and a viewwith respect to an object to be examined in the case of acquiring databy a multislice multiecho method (hereinunder an image at the same slicewhich has a different echo time will be defined as a slice in a broadsense), for example, by the Fourier method by means of such aconventional NMR imaging apparatus. In FIG. 5, the reference numeral 15represents an object to be examined, and the symbol m represents thenumber of slices and n the number of views. Ordinarily, k items ofsample data are acquired in one measurement, and such measurement isrepeated j times per view in order to obtain the average measured data.

The operation of the NMR imaging apparatus is as follows. In the actualapparatus, the number m of slices is typically 32 and the number j ofmeasurements for averaging is typically 8, but hereinunder it is assumedthat the number m of slices is 2, the number n of views 256, the numberof items of data 256, and the number j of measurements 2, for thepurpose of simplifying the explanation.

When the sequence controller 6 drives the power source and driver 1 at aconstant timing on the basis of a command from the CPU 7, the probe head4 is energized and the current of the gradient magnetic filed coil 5 isturned on and off, as is required for measurement of an NMR signal. Itgoes without saying that a uniform and static magnetic field has beengenerated in advance by the static magnetic field coil 2. After the baseband of an NMR signal received by the probe head 4 is converted into anaudio frequency by the preamplifier and detector 3, the NMR signal issupplied to the A/D converter 14.

The pulse sequence at this time is carried out in such a manner as isindicated by (a), (b), (c) and (d) in FIG. 6. (a), (b), (c) and (d) inFIG. 6 represent the timing for energizing the probe head, and thetimings for applying gradient magnetic field in the directions of x, yand z, respectively. In correspondence with these operations, an NMRsignal such as a free induction decay signal (FID signal) indicated by(e) in FIG. 6 is detected.

NMR signals E_(m) /(#n, j) are acquired in the order of detection asshown in the column A of FIG. 7, and are stored in the DISK 10 in thatorder. The measured value of an NMR signal E_(m) (#n, j) is composed ofk items of sample data. The data stored in the DISK 10 are arranged insuch a manner that the data measured at a first time at one view arearranged in the order of slices, and the data measured at a second timein the same view are next arranged in the order of slices, and sucharrangement is repeated for every view. Therefore, measured data arevery complicated with respect to a slice, and the data on the same sliceare not collected at the same place. The CPU 7 calculates {E₁ (#1, 1)+E₁(#1, 2)}/2 to obtain the average value of the data E₁ (#1, 1) on theslice 1 measured at first time at a view 1 and the data E₂ (#1, 2) onthe slice 1 measured at a second time at the view 1. The average valueE₁ (#1) obtained is re-stored in the DISK 10 as a raw data on the slice1 at the view 1. The CPU 7 executes a similar averaging calculationabout all the data on each slice at all the views, and all the averagevalues obtained are subsequently re-stored in the DISK 10. Thus, theaveraged raw data E₁ (#1), E₂ (#1), . . . E₁ (#256), E₂ (#256) arere-stored in the DISK 10 in the order shown in the column B of FIG. 7.In this state, the arrangement of the data is still complicated withrespect to a slice.

When the image is reconstructed, the CPU 7 picks up the data E₁ (#1), E₁(#2), . . . E₁ (#256) on the slice 1, for example, from the data storedin this state, as shown in the column C of FIG. 6, and the AP 11reconstructs the image of the slice 1 on the basis of the collecteddata. The reconstructed image is displayed on the CRT 9. When the imageof the slice 2 is reconstructed, the CPU 7 and the AP 11 execute asimilar processing on the data E₂ (#1), E₂ (#2), . . . E₂ (#256).

Such a conventional NMR imaging apparatus is disadvantageous in thatsince measured data are stored in a large-capacity memory in the orderof acquisition in a complicated state with respect to a slice, and animage is reconstructed by picking up raw data on the corresponding slicefrom data stored in such a state, a heavy load is applied to the CPU 7or the AP 11, so that the speed of reconstruction of a image is lowered.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide an NMRimaging apparatus which is provided with an improved data acquiringdevice and which is useful for enhancement of the speed ofreconstruction of an image.

An NMR imaging apparatus according to the present invention ischaracterized in that a raw data memory 18 which is provided for storingraw data for the maximum number of slices that can be acquired in onescanning separately from the memory of a computer system, and an addressconverter 19 for converting the addresses of the measured data suppliedsubsequently in accordance with the sequence of the multislice multiechomethod and for storing the measured data in the raw data memory 18 inthe arrangement different from the order of acquisition are disposed ina data acquiring device, so that a data block for each slice is formedin the raw data memory.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of an embodiment of the present invention;

FIG. 2A shows the sequence for measuring an NMR signal in an embodimentof the present invention;

FIGS. 2B and 3 are memory maps showing the stored state of raw data;

FIG. 4 shows the structure of a conventional apparatus;

FIG. 5 is an explanatory view of data acquisition by a multislicemethod;

FIG. 6 shows the sequence for measuring an NMR signal by a multislicemethod; and

FIG. 7 is an explanatory view of the data acquisition and dataprocessing in the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be explained in detail hereinunder withreference to the accompanying drawings.

FIG. 1 shows the structure of an embodiment of the present invention. InFIG. 1, since the same numerals are provided with the elements which aresame as those in FIG. 4, explanation thereof will be omitted. Thereference numeral 16 represents a data acquiring device including theA/D converter 14, and is provided with an averager 17 for averaging theoutput data of the A/D converter 14, a raw data memory 18 to which theoutput data of the averager 17 are supplied, and an address converter 19for supplying an address signal to the averager 17 and the raw datamemory 18. The address converter 19 converts the addresses of the outputdata of the A/D converter 14 before storing them in the raw data memory18, so that the data which are supplied in a complicated order withrespect to a slice are stored in each slice area which is provided inthe raw data memory 18 in accordance with the slice numbers of therespective data. The A/D converter 14, the averager 17 and the addressconverter 19 are controlled by the sequence controller 8. The data ofthe raw data memory 18 are transferred to the DISK 10 through the I/O12.

The operation of the embodiment will now be explained. It is alsoassumed that the number m of slices is 2, the number n of views 256, thenumber of items of data 256, and the number j of measurements 2, as inFIG. 4.

The sequence controller 8 drives the power source and driver 1 under thecontrol of the CPU 7 in the same way as in the prior art. That is, thefollowing steps (a) to (e) are executed.

(a) An NMR signal is measured at the view No. 1 and the slice 1.

(b) An NMR signal is measured at the view NO. 1 and the slice 2.

(c) The steps of (a) and (b) are repeated for a second time.

(d) The steps of (a), (b) and (c) are subsequently repeated with 1 beingadded to the view number.

(e) The step (d) is repeated until the view number reaches 256.

NMR signals indicated by (a) in FIG. 2A are generated by theabove-described operation, and are subsequently detected by the probehead 4. The detected NMR signals are converted into digital data by theA/D converter 14 and supplied to the averager 17 in the form of a trainof data indicated by (b) in FIG. 2A, namely, E₁ (#1, 1), E₂ (#1, 1), E₁(#1, 2), E₂ (#1, 2). . .

The averager 17 outputs a first measured data as it is and writes itinto the data memory 18, but when the averager 17 fetches a second orlater measured data, it reads the preceding measured data from the rawdata memory 18 and outputs the average value of the preceding measureddata and the data fetched at that time. Such operation of the averager17 is executed under the control of the sequence controller 18. Theaddress for writing and reading measured data is supplied from theaddress converter 19. Therefore, when the data E₁ (#1, 1) and E₂ (#1, 1)are subsequently input to the averager 17 in the steps (a) and (b), theyare output as they are, and are stored in the address for data E₁ (#1)in a slice 1 data area and in the address for data E₂ (#1) in a slice 2data area, respectively, as shown in FIG. 2B, on the base of theaddresses assigned by the address converter 17. When the data E₁ (#1, 2)and E₂ (#1, 2) are input to the averager 17 in accordance with theoperation in the step (c), the respective preceding data E₁ (#1, 1) andE₂ (#1, 1) are input from the respective area synchronously with theinput of the respective data E₁ (#1, 2) and E₂ (# 1, 2), and {E₁ (#1,1)+E₁ (#1, 2)}/2 and {E₂ (#1, 1)+E₂ (#1, 2) }/2 are calculated to obtainthe respective average values. The results of calculation are output andthese output data are re-stored in the address for data E₁ (#1) in theslice 1 data area and in the address for data E₂ (#1) in the slice 2data area, respectively, on the basis of the addresses assigned by theaddress converter 19. In the same way, every time the averager 17fetches the data from the A/D converter 14, the above-describedoperation is repeated in combination with the address converter 19.Accordingly, the raw data memory 18 contains the data E₁ (#1), E₁ (#2),. . . , E₁ (#256) in the slice 1 data area, and E₂ (#l), E₂ (#2), . . ., E₂ (#256) in the slice 2 data area after one scanning, as shown inFIG. 3. In other words, data on each slice which are necessary forreconstruction of an image are stored collectively in the areacorresponding to the respective slice in the raw data memory 18.

As described above, since the averager 17 calculates an average value ata real time, the raw data memory 18 need not have an area for storingthe data measured a plurality of times so long as it is provided with acapacity for storing data for one scanning.

After one scanning, the data in the raw data memory 18 are transferredto and stored in the DISK 10 while maintaining the arrangement in whichdata are collected for each slice. In order to reconstruct an image, theAP 11 reads the data necessary for reconstruction of the image from thearea in which the data on the desired slice are stored, and conducts apredetermined processing. Since the data necessary for reconstruction ofthe image are collected in an area for each slice, it is possible toread the data with high efficiency, thereby enabling the image to bereconstructed with a high speed.

The present invention does not limit the number of views, the number ofslices, nor the order of data acquisition to that of this embodiment.

While the best mode for carrying out the present invention has beendescribed, it will be obvious to those skilled in the art that variouschanges and modifications may be made without departing from the scopeof the following claims.

What is claimed is:
 1. An NMR imaging apparatus having:magnetic fieldapplying means 1, 2, 4 and 5 for applying magnetic fields for measuringan NMR signal to an object to be examined; measuring means 3 and 4 formeasuring said NMR signal produced on said object to be examined; an A/Dconverter 14 for converting a measuring signal output from saidmeasuring means into digital data; and a computer system 6 including asequence controller 8 for controlling said magnetic field applying meansin accordance with a sequence based on a multislice multiecho method (animage at the same slice which has a different echo time is defined as aslice in a broad sense), a memory 10 in which digital data converted bysaid A/D converter is stored, and a central processing unit 7 forcontrolling said sequence controller and reconstructing a tomogram ofsaid object to be examined on the basis of said digital data stored insaid memory; characterized in that said NMR imaging apparatus isprovided with a raw data memory 18 which is capable of storing outputdata of said A/D converter for the maximum number of slices that areacquired at least one scanning and is capable of transferring the storeddata for at least one scanning to said memory of said computer system;and an address converting means for converting the address of outputdata of said A/D converter before said data is stored in said raw datamemory so that a block of raw data for each slice is formed in said rawdata memory.
 2. An NMR imaging apparatus having:magnetic field applyingmeans 1, 2, 4 and 5 for applying magnetic fields for measuring an NMRsignal to an object to be examined; measuring means 3 and 4 formeasuring said NMR signal produced on said object to be examined; an A/Dconverter 14 for converting a measuring signal output from saidmeasuring means into digital data; and a computer system 6 including asequence controller 8 for controlling said magnetic field applying meansin accordance with a sequence based on a multislice multiecho method (animage at the same slice which has a different echo time is defined as aslice in a broad sense), a memory 10 in which digital data converted bysaid A/D converter is stored, and a central processing unit 7 forcontrolling said sequence controller and reconstructing a tomogram ofsaid object to be examined on the basis of said digital data stored insaid memory; characterized in that said NMR imaging apparatus isprovided with an averaging means 17 for averaging digital data outputfrom said A/D converter in accordance with measurements executed aplurality of times at the same slice and the same view; a raw datamemory 18 which is capable of storing output data of said averagingmeans for the maximum number of slices that are acquired at at least onescanning and is capable of transferring the stored data for at least onescanning to said memory of said computer system; and an addressconverting means for converting the address of output data of saidaveraging means before said data is stored in said raw data memory sothat a block of raw data for each slice is formed in said raw datamemory.